Method to measure the relative perfusion of the lungs

ABSTRACT

The invention covers a method to determine the relative lung perfusion image with the fluoroscopic principle. Transmitted X-ray radiation through the lungs of the patient under examination is measured at least during one heart cycle. An image intensifier is used to collect the images which are digitized, taken to the apparatus and further analyzed. The results are the grey scale image and numerical map depicting the lung perfusion distribution.

BACKGROUND OF THE INVENTION

This invention deals with a method to measure the perfusion of the lungsin fluoroscopy in which it is possible to obtain the result as anumerical map which shows the perfusion normalised to the maximum valuein the map.

Many different methods have been and are used to measure and evaluatedifferent parameters of the function of the lungs. Methods possessconsiderable differences both in the performance and other factors likegeneral requirements, pain and inconvenience to the patient, duration ofthe measurements, patient radiation dose.

Pulmonary embolism (PE) refers to the total or partial blockage of bloodcirculation in pulmonary arteries, and the seriousness is connected tothe extend of the blockage. The diagnosis of PE is an importantprocedure in terms of the prognosis of the patient. An non-diagnosed PEmay cause the death of the patient with the probability of about 30%,while when treated with anticoagulation it is 8%. Pulmonary embolism is,therefore, diagnosed based on the information obtained from nuclearmedicine ventilation-perfusion scans, high resolution computedtomography HRCT or pulmonary angiography. The last method, pulmonaryangiography is used to lesser extent due to its invasive nature andoften poor availability. Scintigraphic ventilation-perfusion scans aremost commonly used, but the interpretation of the examination may beambiguous. The recently widely used method is CT imaging with contrastinfusion in the spiral mode.

Information of different methods describe the lung function fromdifferent aspects, therefore different methods complement each other.

BRIEF SUMMARY OF THE INVENTION

The goal of this invention is to set up a method in which a clear imageis obtained of the conditions of lung perfusion. The examination is fastwith no invasive interventions and with only a small radiation dose tothe patient. The goal is achieved as is said to be characteristic to theinvention in the accompanying claims.

The invention is described more accurately in the following using theenclosed drawings:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 depicts a general view of the apparatus in use.

FIGS. 2a and 2 b show the grey scale image and corresponding numericalmap of the perfusion of the lungs of a healthy volunteer made withimaging method of the invention.

FIGS. 3a and 3 b show the corresponding images of a sick patient.

DETAILED DESCRIPTION OF THE INVENTION

The method of the invention is described in the following whilereferring to the figures. FIG. 1 shows the simplified apparatus. Theapparatus 1 is made of the radiation source, for instance an X-ray tube2 with about 100 kVp which emits the radiation beam towards the patient4 lying on the bed 3. The apparatus includes also a filter or filters 6in the usual way. The radiation beam is detected with an imageintensifier tube 5 and the gathered data is collected with for example aPersonal Computer (PC) device for further handling and analysis.

During the imaging the patient has to hold his breath, at least duringone heart cycle. Also a patient in a fairly poor health condition canmake this. As mentioned earlier no interventions are performed to thepatient, the method is fully non-invasive, no radioactive isotopes orcontrast media is used. The method is fast and advantageous in use.

Describing in greater detail the apparatus and the method function inthe following way. High voltage of about 100 kVp with a fixedfluoroscopic mA-value and a copper filter of approximate thickness 1 to3 mm are used in imaging. High voltage is varied depending on patientsize. Otherwise fluoroscopic parameters are kept constant during thestudy. The varying X-ray transmission caused by pulmonary perfusion arerecorded.

The patient dose is approximately equal to a conventional chest X-rayexamination. The result of the imaging is perspicuous, in nearlycomplete non-visualisation of ribs, but clavicles may on the other handdiminish somewhat the visualisation of lung apices.

A series of digital images usually 50 or 100 with 256 grey shades arecollected at 12.5 or 25 Hz. Generally the matrix size of 384×288 isused.

The images are analyzed in different ways after the collection of thedata. The original and subtracted image series and parametric imageslike the images of the amplitude, asynchrony and perfusion are producedand animated, and quantitative curves and values from user selectedROI-areas are calculated. In the subtraction mode the average image of aseries consisting of one or several perfusion cycles can be used. Thedevelopment and analysis of perfusion images collected while holding thebreath requires dedicated, careful analysis, because intensity changescaused by perfusion and partly by the movements of blood vessels andlung tissues are quite small.

The aim of this invention is to measure quantitatively local lungpulsation in PA projection. The perfusion (blood pulsation) signal inthe lung parenchyma is weak, weakest in the periphery, being in therange of 1-2 units when the image noise is 20 approximately 0.2-0.5units. The measured perfusion images demonstrate local and temporalchanges in the X-ray transmission through the lungs, caused by changesof the blood content and corresponding density changes of the lungparenchyma. Local variations in user selected subareas are processed.Cycling pulses are seen in curves from selected ROI-areas and faintlyalso in the animation of subtracted images. The phase differencesbetween different ROI areas are minor facilitating the followinganalysis in normal subjects.

The perfusion images are analyzed interactively roughly in the followingway. The analysis starts by animating the measured images to ascertainthat the diaphragm of the patient does not move. The program drawsborder lines to the lungs which can also, when necessary, be correctedmanually. The user selects a ROI area anywhere in the middle of eitherlungs. The user marks two or more minima points in the therebycalculated perfusion curve which may be the original or for instance athree-point time averaged curve.

The interpreter selects the size of a submatrix usually between 1×1-6×6pixels, possibly up to 16×16 which will be used in the calculation oflocal pulsations from the entire image series. A parametric perfusionimage is constructed of the summed areas of these pulsations using theselected time points. The normalized grey scale perfusion image andcorresponding numerical map(s) are displayed and printed out. Thehighest value found inside the lungs in the numerical map is normalizedto 100 units. ROI-technique can further be utilized to analyze andquantitatively compare lung regions.

The pulsation in the aorta which is at the same phase with lungpulsation must in most cases be excluded to facilitate the propernormalization of the perfusion images. Minor phase differences betweencentral and peripheral ROI-areas in the lungs do not generally disturbthe analysis.

The basic idea, as described above, in calculating pulsating blood flowin the lungs is to equalize it with the summed area in one or morecycles in measured image series when the patient has been able to holdhis breath. If the analysis is subjected to the entire image area,perfusion images with more disturbing noise in the surroundings aroundthe lungs may be obtained.

The purpose of the submatrix analysis is to obtain a perfusion numericalmap in which the distribution and amount of pulsating perfusion caneasier be comprehended. It has been noted that the analysis based on asubmatrix, for instance 6×6, results in an illustrative appearance ofperfusion intensities while that of for example 16×16 analysis is toocoarse.

In FIGS. 2a, 2 b and 3 a, 3 b the grey scale perfusion images (figuresa) and the numerical maps (figures b) analyzed by this invention areshown. It can clearly be appreciated that the lung perfusion of thehealthy subject is smooth while the corresponding distribution of lungperfusion of the sick patient (clinically and scintigraphically provenPE ) is very irregular and heterogeneous.

It has also been found that the lung perfusion in a normal volunteervaries little from one cycle to another. Patients with disturbance inthe lung perfusion may on the other hand have uneven lung perfusioncycles, extra heart beats and other disturbances which may also bediagnosed with these measurements.

In grey shade perfusion image some central lung blood vessels and thetraces of their movements can be seen. The perfusion signal even frombehind the heart is detectable in slim patients.

Other oblique projections but the PA one can be taken to yieldadditional localizing information and to clarify other influencingfactors.

The imaging method of this invention is very reliable when central areasof the lungs are measured. Areas in the lung periphery are thinner andcontain less blood. Therefore the measured signal is weak and may ariseproblems in the interpretation. Uncertainty is also caused by theaveraging analysis and the vagueness to determine the curve minimapoints based on the submatrix calculations. Generally speaking borderareas have smaller significance in determining lung perfusion and thereliable measurement of the central areas obtained with this inventionis sufficient for the diagnosis.

The method of this invention has been tested in different patientexaminations, and it has been found to function properly. The analyzedimages are clear and illustrative.

The method according to the invention can be modified in many ways whilekeeping within the inventive basic idea and the scope of the appendedclaims.

What is claimed is:
 1. Method to determine lung perfusion with thefluoroscopic principle, characterized in that the transmitted X-rayradiation of the patient under examination is measured during at leastone heart cycle using an image intensifier, the signal of the imageintensifier is digitized, the signal is taken into a Personal Computer(PC) based device and analyzed.
 2. Method according to claim 1,characterized in that perfusion curve from a user selected region ofinterest is calculated, two or more minima points are selected from thecurve, the size of the submatrix in use is selected and used for thecalculation of the amount of perfusion in each submatrix utilizing theentire image series and the sums of the signal from the images betweenthe minima points are used to make up the perfusion image.
 3. Methodaccording to claim 2, characterized in that the obtained grey scaleimage of relative lung perfusion is normalized and further processedinto a numerical map.
 4. Method according to claim 1, characterized inthat it is used X-ray radiation with high voltage and a thick copperfilter.
 5. Method according to claim 4, characterized in that the highvoltage is about 100 kVp and the thickness of the copper filter is fromabout 1 mm to about 3 mm.